The present invention relates to a portable terminal for mobile communication and particularly to a high-power portable terminal having means for reducing an antenna standing-wave ratio to protect an output amplifier from a reflected wave caused by an antenna.
Conventionally, many portable terminals for mobile communication that employs a communication satellite or the like use a high-frequency, high-power time division duplex system (TDD system).
FIG. 1 is a diagram showing an exemplary structure of such a conventional portable terminal. As shown in FIG. 1, the conventional portable terminal has an antenna 1, a transmitter/receiver switch (Tx/Rx switch) 2, a part of microphone 11, a power amplifier 12, an isolator 13, a low-pass filter 14, a part of earphone 21, a low noise amplifier 22 and a band-pass filter 23. A controller, not shown, is also provided for various controls of the portable terminal such as dialing control.
In transmission, a speech signal input from a microphone (not shown) of the part of microphone 11 is converted and modulated into an electric signal. The output signal is amplified by the power amplifier 12, passed through the isolator 13 and filtered by the low-pass filter 14 so that excess high-frequency components will be eliminated from the output signal. Then the output signal is passed through the Tx/Rx switch 2 and transmitted from the antenna 1.
In reception, signals received by the antenna 1 are passed through the Tx/Rx switch 2. Then only a given frequency band of signals is selected by the band-pass filter 23, amplified by the low noise amplifier 22, demodulated in the part of earphone 21 and converted to a voice at an earphone (not shown).
When using high frequencies, a portion of the output signal from the power amplifier 12 is reflected by the antenna 1 and interference of the reflected wave may form a standing wave. The standing-wave ratio (maximum standing wave/minimum standing wave) at the antenna 1 is standardized to be within a fixed range. However, when the antenna 1 has been damaged or electromagnetic conditions have changed, an unusually large reflected wave may be sent back from the antenna 1 to the output end of the power amplifier 12 to cause an error or a failure in the power amplifier 12.
The isolator 13 is an element acting to pass signals only in the forward direction, which is inserted for absorbing reflected waves from the antenna 1 and protecting the power amplifier 12. As shown in FIG. 1, the use of the Tx/Rx switch 2 for changing over between transmission and reception permits the reflected waves to be directly sent to the output end of the power amplifier 12 when an abnormal condition such as failure of the antenna 1 has occurred. In such a case, the danger of error in the power amplifier 12 becomes large and therefore the isolator 13 is required for avoiding this error.
As shown in FIG. 2, another conventional portable terminal uses a circulator 3 instead of the Tx/Rx switch 2, which carries the reflected waves from the antenna 1 to the receiving side. In this technique, when transmission power is small (when the output of the power amplifier 12 is small), neither a protective circuit for the low noise amplifier 22 nor the isolator 13 is required. However, when the transmission power is large, a protective circuit 24 needs to be provided for protecting the low noise amplifier 22. Since the protective circuit 24 sends back the reflected waves from the antenna 1 through the circulator 3 to the transmitting circuit, the isolator 13 also needs to be connected to the output end of the power amplifier 12 for protecting the power amplifier 12.
The following drawbacks are present in the above conventional portable terminals.
(1) The isolator 13 is expensive and the use of the isolator 13 increases the total cost.
(2) The parts are too large to use in a compact portable terminal.
(3) Since the isolator 13 increases losses on the transmitting side from the antenna 1 to the power amplifier 12, the power amplifier 12 must be a high power type and this also increases the total cost. Further, such a high-power amplifier deteriorates distortion characteristics of the output signal.
(4) Increased power consumption of the power amplifier 12 reduces battery working time and hence duration of a call.
(5) Although a large-sized battery is required to secure the duration of a call, such a battery is too large to use in the portable terminal.